This invention relates to an opposite phase detecting device which detects an opposite phase condition of a three-phase AC power supply for alternating current electrical equipments.
Three-phase induction motors have widely been employed as drive sources of large-scaled electrical equipments such as conveyors, blowers and pumps and are supplied with a three-phase AC power supply.
Conventionally, in the condition that the three-phase AC power supply is normally connected to the three-phase induction motor in the normal order of phases, the three-phase induction motor is forwardly rotated to thereby normally driving the electrical equipment. However, in the case where the three-phase AC power supply is connected to the three-phase induction motor by mistake in an opposite phase condition, the three-phase induction motor is reverse rotated, resulting in an abnormal drive of the electrical equipment. Accordingly, occurrence of the opposite phase in the three-phase AC power supply, when occurs, needs to be detected with certainty at the starting of the three-phase induction motor, thereby taking a suitable countermeasure.
Japanese Laid-open Utility Model Application (Kokai) No. 62-70631 discloses one of conventional opposite phase detecting devices. In the disclosed device, each phase voltage of the three-phase AC power supply is half-wave rectified by a waveform shaping section and thereafter, a trigger pulse having a small width generated at the rising zero cross point of the half wave voltage by a pulse forming section. Two flip-flops and AND circuits are employed for determining the order of phases that the trigger pulses are generated.
The above-described prior detecting device has a problem that the number of circuit elements are increased though the opposite phase detecting operation thereof has a certain reliability.